Wiring RCD FCU


Postby chris_on_tour2002 » Sun Mar 08, 2009 9:56 pm

Hi folks,

i am putting in an RCD fused spur unit to feed a dry underfloor heating system in a downstairs cloakroom. unfortunately i have mislaid the wiring instructions for the device and hope someone can clarify.

the RCD unit is not actually going to run from a spur, instead i have removed a single socket plate and replacing with the RCD unit, hence it will be part of the ring main.

so i have feed and return terminals for the load and supply. it's the earth terminals i'd like to clarify.

there are two on the unit - one marked E with the earth symbol the other marked SE also with the earth symbol. i assume that i don't simply bridge the two, one is the earth connector for the load the other for the supply? logic may dictate that the SE terminal is 'Supply Earth'?

also the back box is steel so of course needs earthing. i understand that with this type of RCD unit it matters whether it is connected to earth on the load or supply side of the unit. i would assume the supply side as it goes back to the board. or am i getting confused as it's earthing it should all be interlinked?

one more thing, its a 140 watt heating system. by my calculations this will only require a 3 amp fuse. 140W/230v = 0.6A so would a 1 amp fuse be better?

cheers,
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Postby sparx » Mon Mar 09, 2009 6:15 pm

Hi,
Both earth terminals should be linked internally so no matter which way round connected, BTW absolutely no requirement to 'pig-tail' earth to back box if wiring is twin/E,
regards SPARX
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Postby chris_on_tour2002 » Tue Mar 10, 2009 12:18 am

thanks for that sparx - i called the manufacturer for advice this morning before i read your post. turns out that the second earth terminal is only there in case a "clean earth" is required. could you tell me what this is please? i would assume that means the appliance has a dedicated earth, or one that goes straight back to the board and not via a larger circuit ie ring main, is this correct? in any case i was told to bridge the terminals on the unit for my purposes.

also why is there no need to 'pig-tail' earth backboxes for T&E?
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Postby sparx » Tue Mar 10, 2009 3:48 pm

Hi again, C-o-T,
you are correct about'clean earth' it's for use of computors/radio equipment in certain locations where as you say an independant earth is run in direct from intake position routed away from other wiring so not likely to suffer from interference (electrical).
Back boxes:
Under current regs it is still permissable to run in conduit/trunking system and use the metal work as your CPC (earth), however if doing so it is required to 'pigtail' FROM box TO front plate of accessory to bring CPC forward.
If wiring in a CPC, either with singles in conduit or as part of TW/E cable it is NOT required to 'pigtail' back to box, providing at least one of the fixing lugs is not moveable on the side of the accessory that has an earthed screw hole, most fittings have both lugs earthed anyway.
For the benefit of those Niccy contractors on here who have been lied to,
see page 131, regs: 543.2.2.(iv), 543.2.5, 543.2.4, 543.2.7 also just confirmed by both ECA & NAPIT published articles.
Why add to an already cramped back box with unneccessary bits of wire?
Also the worst practice of all IMHO is to take one ring CPC to front plate term. & one to back box & link between as some will do, asking for trouble!
Lets see if anyone else has a different idea, backed by regs not 'wholesaler's counter gossip,'
regards SPARX
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Posts: 2166
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